1. Field of the Invention
The present invention relates to a variable impedance circuit for high-frequency applications, for uses such as automatic gain control.
2. Description of the Related Art
In the prior art, PIN diodes have been used as variable impedance elements for high-frequency applications. However with a PIN diode it is necessary to pass a DC current through the diode, with the DC current level being varied to produce variations in the diode impedance.
Field effect transistors (FETs) have also been used as variable impedance elements, for example as described in Japanese Patent Application No. 3-206889. In that case, the impedance of the channel of an FET is varied by altering a DC voltage applied to the gate electrode of the FET, i.e. with the impedance between the drain electrode and source electrode of the FET being thereby varied.
The PIN diode has the disadvantage that it is necessary to pass a significant level of current through the diode, with resultant power consumption. In some applications in which power consumption must be minimized, this is a serious disadvantage. If a FET is used as a variable impedance element as described above, there is the disadvantage that the relationship between drain voltage and channel current becomes very non-linear, when the gate voltage is brought close to the channel pinch-off condition. As a result, problems such as increased intermodulation distortion and deterioration of the high-frequency input/output characteristics (i.e. an excessively high VSWR figure) will arise.
These disadvantages will be made clear by examining the DC characteristics of an n-channel depletion-type GaAs MES FET, which could be used as a high-frequency variable impedance element with the source electrode connected to ground potential, an input signal voltage applied to the drain electrode, and a variable DC control voltage applied to the gate electrode to thereby vary the channel impedance. FIG. 1A shows a circuit containing such a FET, set up to measure the DC characteristics. The FET is assumed to have a threshold voltage of -0.4 V, an a gate width Wg of 200 .mu.m. In FIG. 1A, the gate voltage Vg is varied as a parameter, applied through a resistor RB of value 2 k.OMEGA. (inserted to prevent any significant level of current from flowing into the channel from the gate electrode), with the resultant relationships between applied drain voltage Vd and resultant drain current Id being measured. The characteristics obtained in this way are shown in FIG. 1B.
As shown in FIG. 1B, there is a substantial deterioration of linearity of the Vd/Id characteristics when the gate voltage Vg is approximately -0.4 V and the drain voltage Vd is close to 0 V. This is due to the fact that when Vd is greater than 0 V, the channel of the FET is in the pinch-off condition, or is close to the pinch-off condition, whereas when Vd is less than 0 V, the channel becomes opened. Hence, there are large changes in Id when Vd is close to 0 V and the gate voltage is close to the threshold voltage.
Such a high degree of non-linearity of the Vd/Id characteristics results in a high level of intermodulation distortion and deterioration of the input/output characteristics, when such a FET is used as a high-frequency variable impedance element.
To overcome that problem of non-linearity of the characteristics when the value of Vd is close to zero, it would be possible to use a combination of a pair of complementary FETs in a variable impedance element functioning as a variable impedance element. However it is difficult to produce complementary FETs of a type which are suitable for high-frequency operation, by a practical method of manufacture, i.e. with n-type and p-type FETs being manufactured in the same process, in which mutually adjacent FETs are produced using the same mask patterns. In addition, it is difficult to form complementary FETs of a type which are suitable for such high-frequency applications and which are also closely matched in electrical characteristics. Hence it is difficult to actually implement such a variable impedance element using complementary FETs.